An antenna circuit of a mobile communication terminal allows an antenna to transmit or receive predetermined radio signals.
In order to optimize the transmit/receive radiation performance of the antenna, impedance matching must be exactly performed.
Therefore, the antenna circuit includes capacitors and inductors, and the impedance of the antenna is matched in the optimal status by adjusting values of the capacitors and the inductors.
In general, the impedance matching of the antenna is achieved in a state that the mobile communication terminal is positioned in a free space.
Meanwhile, the mobile communication terminal may be used by a user in a state that the user handholds the body of the mobile communication terminal and adheres a speaker to the ear of the user due to the mechanical characteristic of the mobile communication terminal, or may be used by the user through an ear-phone in a state that the body of the mobile communication terminal is put into a pocket or a bag of the user.
As the user uses the mobile communication terminal by gripping the mobile communication terminal with the hand of the user and closely touching the speaker to the ear of the user, or as the user uses the mobile communication terminal put into the pocket or the bag, the conditions for the impedance matching of the antenna may vary, so that the transmit/receive radiation performance of the antenna subject to impedance matching in the free space may be degraded.
Therefore, the mobile communication terminal employs an adaptive tuning antenna circuit to automatically adjust the impedance of the antenna when the conditions for the impedance matching of the antenna vary, so that the antenna has the optimal transmit/receive radiation performance.
The adaptive tuning antenna circuit must detect the impedance variation of the antenna to maintain the transmit/receive radiation performance of the antenna in the optimal state.
To this end, the adaptive tuning antenna circuit includes a coupler to detect reflected power and forward power output from the coupler, and to change the capacitance of a variable capacitor according to the reflected power and the forward power, so that impedance matching can be performed.
According to the related art, as shown in FIG. 1, after sequentially scanning the whole variable region of the variable capacitor, a point representing the best RF performance is selected and adapted, thereby performing impedance matching.
However, if the whole region of the variable capacitor is scanned as described above, a point representing the inferior RF performance is inevitably scanned, so that the transmit/receive radiation performance may be degraded.